Incendio

From UnknowableWiki

Incendio is a spell that conjures a small fire, used mainly on fireplaces. Arthur casts it on the Dursleys' fireplace, allowing Fred, George, Ron and Harry to use the Floo Network (GF4).

It is also used by a Death Eater to set Hagrid's hut on fire, after the Battle of the Astronomy Tower (HBP28).

Ginny Weasley mentions that she, along with Neville Longbottom and Ron, unsuccessfully tried to cast the spell against the Peruvian Instant Darkness Powder used by Draco Malfoy to allow Death Eaters to leave the Room of Requirement unnoticed (HBP29).

Even though the incantation is not uttered, it is possible to assume that this is the spell Hagrid uses to light the fireplace in the hut to which the Dursleys travelled in their quest to escape Harry's Hogwarts letters (PS4).

Voldemort presumably uses this spell to set the Sorting Hat on fire, placing it on top of Neville Longbottom's head (DH36).